A bag with side folds is known from European Patent No. 0,123,784,B, the bottom of which is formed by first folding a strip of the front wall together with a side fold around a first folding line running transversely to the longitudinal axis, while the corresponding strip of the rear wall remains in place. This is possible since cuts are provided in an edge area between the rear wall remaining in place and the side fold adjoining it. The strips of the rear wall remaining in place are folded once again around the outer edge of the folded strip and attached in this position on the outer fold of the sack or bag. Such a sack or bag has the disadvantage that the bottom portion is not completely tight and leak-proof, so that, for instance, fine-grained flowable bulk products can leak from the bag. This lack of tightness is caused by the previously mentioned cuts in the side area.